1. Field of the Invention
The present invention relates to mobile communication terminal equipment, a control method therefor, and a recording medium on which a control program therefor is recorded and, more particularly, to an improvement in a cell detection method in mobile communication terminal equipment using CDMA (Code Division Multiple Access).
2. Description of the Prior Art
In a cellular mobile communication system, a wide service area is constituted by a plurality of cells each covering a relatively small range, and a mobile station that moves in this service area communicates with a base station installed in each cell. As the mobile station moves, therefore, the base stations installed in the respective cells which are optimal for communication sequentially change. For this reason, in the mobile communication system, selection of an optimal base station for communication, i.e., so-called cell detection (cell selection), must be performed.
Whether cell detection is accurately performed is greatly influenced by a subscriber capacity, communication quality, and the like. That is, when a remote cell is erroneously selected, both the mobile station and the base station perform transmission with larger transmission power than when a correct cell is selected. This increases interference in other stations, and the signal power to interference noise power ratios in the other stations decrease, resulting in a deterioration in communication quality. In addition, as the interference increases, the number of stations that can simultaneously communicate decreases, resulting in a decease in subscriber capacity.
A conventional cell detection method will be described below. In a CDMA cellular phone system, each cell (base station) has a unique scramble code. A mobile unit (mobile station) as mobile communication terminal equipment detects (searches) such a scramble code to perform cell detection (base station detection), i.e., specify a scramble code group. FIG. 1 shows the frame format of a cell search radio channel used in this case.
As shown in FIG. 1, a radio frame is constituted by the 1st to 15th time slots (to be simply referred to as slots hereinafter), and each slot is constituted by PCCPCH (Primary Common Control Physical Channel) and SCH (Synchronization Channel). PCCPCH is spread by a spreading code common to all the cells, and is further spread by a scramble code unique to the cell. This scramble code has a radio frame period (10 msec).
SCH is obtained by multiplexing P-SCH (Primary SCH) and S-SCH (Secondary SCH). P-SCH is spread by a spreading code (first search code) common to all the cells but is not scrambled. This first search code is a pattern common to all the slots. S-SCH is spread by a spreading code (second search code) determined by a scramble code group to which the scramble code used for PCCPCH belongs, but is not scrambled. These second search codes have different patterns for the respective slots in a radio frame. There are 32 scramble code groups. One group includes 16 scramble codes. That is, there are 32 types of second search codes.
A mobile unit performs a cell search by receiving this radio frame from a base station in accordance with the flow chart of FIG. 2. More specifically, an SCH portion is despread by using the first search code (known) common to all the cells (steps S131 and S132) to detect P-SCH (step S133). That is, the start timing of slots is recognized, and slot synchronous processing is performed (step S134). Note that the start timing of the frame cannot be recognized.
All the 32 types of second search codes are used to despread the SCH portion (steps S135 and S136) to detect S-SCH. In this case, a scramble code group is specified from the second search code having the largest correlation value (steps S137 and S138). Since the second search codes have different patterns for the respective slots, the start timing of the frame can be simultaneously recognized, thus performing frame synchronous processing.
Subsequently, PCCPCH is despread by using all the 16 types of scramble codes that belong to the specified scramble code group and the PCCPCH spreading code (known) common to all the cells, thereby detecting correlation values and specifying a scramble code having the largest correlation value. The relationship between scramble code groups and scramble codes will be described. For the sake of descriptive convenience, assume that the total number of scramble codes is 100. In this case, if the 100 scramble codes are used one by one in cell search, it takes much time. For this reason, the scramble codes are formed into groups each including 10 scramble codes as follows:
scramble codes 1, 2, . . . , 10 →scramble code group 1scramble codes 11, 12, . . . , 20 →scramble code group 2scramble codes 21, 22, . . . , 30 →scramble code group 3. . .scramble codes 91, 92, . . . , 100 →scramble code group 10
The base station transmits a cell detection search code (equivalent to a second search code) corresponding to a scramble code group to which the self-scramble code belongs. As described above, the mobile station detects this cell detection search code to specify the corresponding scramble code group, and knows a scramble code unique to the base station (cell search) by using 10 scramble codes belonging to this group. This makes it possible to shorten the cell search time.
Since despread processing needs to be performed by using all the scramble codes belonging to the scramble code group specified by the above method, it takes much time to perform a cell search. In order to shorten the cell detection time, a plurality of correlators may be concurrently operated. In this method, however, the circuit size increases accordingly.
As conventional techniques for performing a cell search in a short period of time, the techniques disclosed in Japanese Unexamined Patent Publication Nos. 7-298332 and 7-312771 are available. According to the former technique, base stations notify a mobile station of pieces of neighboring cell monitoring code information, and the pieces of notified cell code information are sequentially scanned in a predetermined order of priority, thus performing a cell search.
In this method, a cell search is performed depending on pieces of neighboring cell monitoring code information from base stations regardless of the movement history of the mobile station itself. Therefore, this method is effective in a cell search in handover operation, but cannot shorten the cell search time at power-on. In addition, base stations must transmit neighboring cell monitoring code information.
According to the latter technique (Japanese Unexamined Patent Publication No. 7-312771), a search is made for the cell detection search code of the cell with which a mobile station has communicated most recently, or a search is made in the order of priority corresponding to the visiting cell history of a mobile station. This method is effective in a cell search at power-on, but is not effective in a cell search in handover operation.